1. Field of the Invention
The present invention relates to a method of manufacturing a capacitive electromechanical transducer which is used as a capacitive micromachined ultrasonic transducer or the like.
2. Description of the Related Art
In recent years, a capacitive electromechanical transducer manufactured through a micromachining process has been researched actively.
A general capacitive electromechanical transducer has a lower electrode, a vibrating film, which is supported to keep a given distance from the lower electrode, and an upper electrode, which is disposed on a surface of the vibrating film.
This capacitive electromechanical transducer is used as, for example, a capacitive micromachined ultrasonic transducer (CMUT).
This kind of capacitive micromachined ultrasonic transducer has a feature that a light-weight vibrating film is used to transmit and receive ultrasonic waves, and that it is easy to obtain a capacitive micromachined ultrasonic transducer that has excellent broadband characteristics in liquids as well as in the air. The CMUT enables more accurate diagnosis than conventional medical diagnosis, and is therefore beginning to attract attention as a promising technology.
Next, the operation principle of the capacitive micromachined ultrasonic transducer is described. To transmit ultrasonic waves, a DC voltage superimposed with a minute AC voltage is applied between the lower electrode and the upper electrode. This causes the vibrating film to vibrate, thereby generating ultrasonic waves.
When ultrasonic waves are received, the vibrating film is deformed by the ultrasonic waves, and hence the deformation causes a capacity change between the lower electrode and the upper electrode, from which signals are detected.
In the capacitive electromechanical transducer, theoretical sensitivity is in inverse proportion to the square of the distance between its electrodes (gap).
A gap of 100 nm or less is necessary to manufacture a highly sensitive transducer, and study has recently been made on the gap in the CMUT of as narrow as 2 μm to approximately 100 nm (and further 100 nm or less).
Meanwhile, a commonly employed method of forming the gap in the capacitive electromechanical transducer involves providing a sacrificial layer that is as thick as a gap between the electrodes, forming the vibrating film on top of the sacrificial layer, and then removing the sacrificial layer.
This and similar technologies are disclosed in, for example, U.S. Pat. No. 4,262,399.
The method involving removing the sacrificial layer described in U.S. Pat. No. 4,262,399 has the following problems.
In this method, the progress of etching determines the cavity size, and hence, when multiple cavities are formed, sacrificial layer pieces for forming the multiple cavities may be etched unequally due to differences in film thickness, film quality, and the like between the sacrificial layer pieces. The resultant problem is uneven cavity size.
One of the possible solutions for this problem is to form each sacrificial layer piece for forming a cavity into the exact shape and size intended in advance, and to remove the sacrificial layer piece through an etching hole which is provided after the vibrating film is layered on top of the sacrificial layer.
However, according to this method, the vibrating film is formed after the patterning of the sacrificial layer, and hence the vibrating film needs to be thicker than a thickness enough to satisfy the step coverage in places where steps are formed by the sacrificial layer, and therefore needs to allow for the thickness margin. Giving the vibrating film a thickness that is thicker than necessary can cause a problem in that vibration characteristics are adversely affected.
In other places than cavities, on the other hand, the vibrating film needs to have a given thickness or more because a parasitic capacitance generated between the upper wiring and the lower wiring via the vibrating film takes a large value and greatly affects the sensor's sensitivity when the vibrating film is thin.
The thickness of the vibrating film thus needs to be controlled to an optimum value by taking into account its influence over the characteristics of the vibrating film which varies depending on where the vibrating film is formed.